AP Chemistry Unit 3 Spectroscopy: Light, Electrons, and Measuring Molecular Amounts

Spectroscopy

The study of how matter interacts with electromagnetic radiation (light), often through absorption or emission of specific energies.

Absorption (of light)

When a substance takes in light of certain energies because the photon energy matches an allowed energy change in the substance.

Emission (of light)

When a substance releases light of certain energies as it undergoes a decrease in energy (e.g., electrons relaxing to lower energy levels).

Electromagnetic radiation

Energy that travels as oscillating electric and magnetic fields; described as waves or as photons.

Quantized energy levels

Discrete (step-like) allowed energies in atoms/molecules; energy changes occur only in specific increments, not continuously.

Photon

A discrete packet of light energy; each photon carries an amount of energy determined by its frequency.

Wavelength ($\lambda$)

The distance between successive wave crests; typically measured in meters (or nm, then converted to meters for calculations).

Frequency ($\nu$)

The number of wave cycles per second; measured in $s^{-1}$ (Hz).

Speed of light (c)

A constant in vacuum (~$3.00 \times 10^8$ m/s) relating wavelength and frequency by $c = \lambda\nu$.

Wave equation ($c = \lambda\nu$)

Relationship showing that wavelength and frequency are inversely related because c is constant (longer $\lambda$ means lower $\nu$).

Planck’s constant (h)

Constant linking photon energy to frequency: $h \approx 6.626 \times 10^{-34}$ J·s.

Photon energy equation ($E = h\nu$)

Equation stating energy per photon is proportional to frequency.

Energy–wavelength relationship ($E = hc/\lambda$)

Equation showing photon energy increases as wavelength decreases.

Ultraviolet–Visible (UV–Vis) spectroscopy

Spectroscopy technique primarily involving electronic transitions; commonly used to measure concentration via absorbance.

Infrared (IR) region

Electromagnetic region that typically causes vibrational transitions; used for functional group identification via bond vibrations.

Nuclear Magnetic Resonance (NMR)

Technique using radio-wave-induced nuclear spin transitions to help determine molecular structure.

Photoelectric effect

Emission of electrons from a metal surface when light of sufficiently high frequency shines on it; supports the photon model of light.

Work function ($\Phi$)

Minimum energy required to eject an electron from a given metal surface; a property of the metal.

Threshold frequency ($\nu_0$)

Minimum light frequency needed for electron emission in the photoelectric effect; defined by $\Phi = h\nu_0$.

Maximum kinetic energy (KEmax)

The greatest kinetic energy of emitted electrons in the photoelectric effect: $KE_{max} = h\nu - \Phi$.

Light intensity (brightness)

Related to the number of photons hitting a surface per second; increases the number of emitted electrons (current) but not KEmax.

Beer–Lambert Law (Beer’s Law)

Linear relationship for many solutions: $A = \epsilon lc$, linking absorbance to molar absorptivity, path length, and concentration.

Molar absorptivity ($\epsilon$)

Measure of how strongly a species absorbs light at a specific wavelength; depends on substance and wavelength.

Transmittance (T)

Fraction of light transmitted through a sample: $T = I/I_0$ (transmitted intensity over incident intensity).

Absorbance (A)

Logarithmic measure of light absorbed: $A = -\log(T)$; unitless and directly proportional to concentration via Beer–Lambert Law.